For workers who may be exposed to chemical vapours the Control of Substances Hazardous to Health Regulations 2002 (amended 2004) and the Dangerous Substances and Explosive Atmospheres Regulations 2002 require risk assessment and control of the hazard. For example, if a cleaning process using solvent is planned, the employer should review possible options and determine if the worker would be exposed to an unsafe level of harmful vapour. If so, steps must be taken to modify the work or protect the worker by ventilation, respiratory protection, etc. As from April 2005, Workplace Exposure Limits (WEL) have replaced the former Occupational Exposure Standards (OES) and Occupational Exposure Limits (OEL). However, these do not directly measure the hazard because in practice this is also determined by the amount of vapour given off by a liquid, which depends upon its vapour pressure. A Vapour Hazard Index is therefore proposed and listed for volatile substances and gases with a WEL. This is the ratio of the saturated vapour pressure to the WEL, which is the amount by which the vapour will exceed the WEL in a confined space. It combines both the toxicological standard and physical properties. It can be used for an initial risk assessment, for example to compare the dangers of alternative solvents. It may also be used in specifying control measures, and is comparable with practice in other countries. For many users, it is suggested that this may be more directly useful than the WEL, so it is therefore proposed that the Vapour Hazard Index should in future be published along with WELs.
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